Method of making electron discharge device cathodes



v lm /e N -roly I WW W ATTQRNEYJ Nov.-l1, 1969 w. HQNEYBALL METHOD OF MAKING ELECTRON DISCHARGE DEVICE CATHODES Filed March 5, .1966 I United States Patent US. Cl. 29-2514 6 Claims ABSTRACT OF THE DISCLOSURE An electron discharge tube thermionic cathode is made by winding a tungsten or molybdenum wire as a helix with spaced turns on a cylindrical former having a surface of alumina. Aluminate material is melted onto the wire winding to cover it, to penetrate the spaces between the turns and to amalgamate with the alumina. Thus the winding is encased in a solid mass when cooled.

This invention relates to cathodes for electron discharge devicesparticularly though by no means exclusively for magnetrons-and has for its object to provide improved cathodes of the kind in which aluminate material constitutes the electron emitting substance. Cathodes of this kind will hereinafter be termed aluminate cathodes.

Aluminate cathodes are well known and present the advantages of high thermionic emission and good robustness, being capable of withstanding considerable maltreatment. Known aluminate cathodes consist essentially of a carrier body of porous tungsten or molybdenuma socalled tungsten or molybdenum sponge-which has been impregnated with aluminate material, usually aluminate of barium, strontium or calcium or a mixture of these aluminates. These known aluminate cathodes have, however, the defect of being diflicult to manufacture and therefore costly, especially in the larger sizes. Moreover they are necessarily indirectly heated and require the inclusion of a high temperature heater to maintain an operating cathode temperature, when in use, of about 950 C. The present invention seeks to provide improved aluminate cathodes which do not present these defects.

According to this invention, a tungsten, molybdenum or tungsten-molybdenum alloy wire is wound helically on the alumina surface of a cylindrical former, and aluminate material is melted on the wire to cover it and penetrate between its turns and to amalgamate with the alumina at the surface of the former. The aluminate material may be aluminate of barium, strontium or calcium or a mixture containing two or more of these aluminates. By this invention the use of a tungsten or molybdenum sponge is avoided and a cheaper cathode, the economy of which is most manifest in the larger sizes, results. Moreover a cathode made in accordance with this invention can readily be designed to be directly heated, if desired, by passing heating current through the supporting wire. The supporting wire may be in mesh form.

In one way of carrying out the invention a tungsten or molybdenum or tungsten-molybdenum wire is wound as a helix on a cylindrical former having a cylindrical surface of alumina, the turns of the helix being spaced a little apart, and the aluminate material is melted on to the 3,477,110 Patented Nov. 11, 1969 wire winding to penetrate the spaces between the turns thereof and encase them, or nearly so. The molten aluminate amalgamates with the alumina surface of the former and, when cooled, encases or nearly encases the Wire in a solid mass. The former may be a ceramic former of alumina or it may be a metal former coated with ceramic alumina. If a directly heated cathode is required the ends of the wire are left projecting out of the mass to serve as connections for passing heater current through the wire. If an indirectly heated cathode is required a heater is provided. For example the former may be made as a sleeve with a heater winding on its inner surface.

In another way of carrying out the invention there is provided a mesh of tungsten or molybdenum or tungstenmolybdenum wire and aluminate material is melted on to the mesh. The wire mesh with its aluminate material carried thereby may, if desired, be sintered on to a ceramic core or former or it may be used as a cathode without such a core or 'former, depending upon the shape and size of the cathode. As before the cathode may be directly heated by using the wire to carry heating current or an independent heater may be provided. Such a heater may, if desired, operate mainly by radiation heating e.g. it could be constituted by a heating wire spaced from the cathode proper and designed to be operated at a suitably high temperature for radiation heating.

The invention can be embodied in cathodes of widely different shapes. Thus, for example, a cathode in accordance with this invention may be cylindrical with its thermionic emissive surface constituted by the cylindrical surface, or it could be approximately disc shaped e.g. with one face of the disc concave and constituting the thermionic emissive surface.

In all embodiments some improvement in thermionic emission can be achieved by mixing a small quantity of powdered tungsten or molybdenum or tungsten-molybdenum alloy with the aluminate material before melting it on to the carrier wire. This metal powder spreads through the aluminate and by increasing the activation increases the emission. Obviously, in the case of a directly heated cathode i.e. one in which the wire carrying the aluminate material is itself fed with heating current, the amount of added metal powder must not be so high as to increase electrical conductivity to the point at which the wire carrying the heating current is effectively shortcircuited.

The invention is illustrated in the accompanying drawings in which:

FIGURE 1 is a sectional view of one form of a directly heated cathode made by the method of this invention;

FIGURE 2 is a sectional view of one form of an indirectly heated cathode made by the method of this invention; and

FIGURE 3 is a sectional view of another form of an indirectly heated cathode made by the method of this invention.

Referring to FIGURE 1 the cathode shown comprises a hollow former 1 made of ceramic (alumina) or of metal coated with alumina. Wound on the outside curved surface of the former is a single layer helix 2 of tungsten, molybdenum or tungsten-molybdenum alloy wire, the turns of the helix being spaced a little apart in the manner indicated. Aluminate material-barium, strontium or calcium aluminate or a mixture containing two or more of these aluminates-is then melted on to the wire helix so that it amalgamates with the alumina 1 and when cooled forms a solid mass 3 encasing the wire or (as illustrated) nearly so. As in other embodiments of this invention some tungsten molybdenum or tungsten-molybdenum powder may be added to the aluminate material before melting it on. If a directly heated cathode is required the ends of the wire may be bent out as shown at 4 to extend from the mass and serve as heating current connections. If an indirectly heated cathode is required a radiating or other heater (not shown) may be provided in manner known per se. The melting on of the material 3 is preferably accomplished by an electric current but other methods of melting the said material may be adopted. In the case of a directly heated cathode good emission can be achieved by passing through the wire 2 a heating current sufficient to raise it to a temperature of about 950 0., whereas, in the case of an indirectly heated cathode having an independent separate heater the required operating temperature of the heater is likely to be about 1600 C. The directly heated cathode thus offers substantial practical advantage.

FIGURE 2 shows schematically a cathode generally like that of FIGURE 1 except that it is indirectly heated. As before 1 is an alumina formerin this case in the shape of a fairly thin sleeve--with aluminate 3 encasing a carrier wire 2 wound on the former. The heater is inside the former and consists of a winding 5 embedded in suitable material 6. The heater connections are marked H and the cathode connection is marked C in FIGURE 2.

FIGURE 3 shows a further variant. This cathode is shown as of the indirectly heated type and is approximately disc shaped with one face of the disc spherically concave and constituting the emissive surface. In FIGURE 3, 11 is an approximately disc shaped ceramic former with one face flat and the other spheriodally concave. A similarly spheroidally shaped mesh 222 of tungsten, molybdenum or tungsten-molybdenum wire is provided on the concave surface of the former and aluminate substance 3 is melted on. A heater winding 5 embedded in suitable material 6 is provided on the flat face of the former.

I claim:

1. A method of making an electron discharge tube thermionic cathode, said method comprising the steps of providing a cylindrical former having a cylindrical surface of alumina, winding a wire of a material selected from the group consisting of tungsten, molybdenum and tungstenmolybdenum alloy onto said former as a helix with spaced turns, melting aluminate material onto said winding so as to penetrate the spaces between the turns of said winding to blanket said winding and to amalgamate with the alumina of said former, and subsequently permitting the whole to cool to a solid mass.

2. A method as claimed in claim 1 wherein the former is of alumina.

3. A method as claimed in claim 1 wherein the former is a metal former coated with alumina.

4. A methodas claimed in claim 1 wherein a small quantity of powdered material selected from the group consisting of tungsten, molybdenum and tungsten-molybdenum alloy is mixed with the aluminate material before melting it on to the wire.

5. A method of making an electron discharge tube thermionic cathode, said method comprising the steps of providing a former having a surface of alumina; applying onto said alumina surface a covering of wire material portions which are spaced a little apart and are electrically connected to one another, said wire material being selected from the group consisting of tungsten, molybdenum, and tungsten-molybdenum alloy; melting aluminate material onto said covering so that said aluminate material blankets said covering, penetrates the spaces between said wire material portions, and amalgates with said alumina surface of said former; and subsequently permitting the whole to cool to form a solid mass.

6. A method as claimed in claim 5 in which said surface of alumina is a part of a concave spherical surface.

References Cited UNITED STATES PATENTS 1,946,603 2/1934 Von Wedel 2925.17 XR 2,175,345 10/1939 Gaides et a1. 29-25.17 2,287,460 6/1942 Wagenhals et al. 29-25.17 2,867,032 1/ 1959 Gehrke et al 2925.17 2,878,409 3/1959 Leir 2925.17 XR 3,117,249 1/1964 Winters 2925.17 XR 3,201,639 8/1965 Levi 2925.17 XR 3,307,241 3/1967 Crapuchettes 2925.17 2,589,521 3/1952 Wheeler et al.

3,195,004 7/1965 Hassett 313340 JOHN F. CAMPBELL, Primary Examiner RICHARD B. LAZARUS, Assistant Examiner U.S. Cl. X.R. 

